Posts Tagged ‘monkeypatch’

The problem: In a test function we want to patch an Environment variable and see if our application handles something related to it correctly. The direct approach for doing this in a test function might look like this:

If we needed to do this several times for test functions we’d have a lot of repetetive boilerplatish code. The try-finally and undo-related code does not even take into account that ENV1 might not have been set originally.

Most experienced people would use setup/teardown methods to get less-repetetive testing code. We might end up with something slightly more general like this:

This avoids repetition of setup code but it scatters what belongs to the test function across three functions. All other functions in the Testcase class will get the service of a preserved environment although they might not need it. If i want to move away this testing function i will need to take care to copy the setup code as well. Or i start subclassing Test cases to share code. If we then start to need modifying other dicts or classes we have to add code in three places.

Monkeypatching the right way

Here is a version of the test function which uses pytest’s monkeypatch` plugin. The plugin does one thing: it provides a monkeypatch object for each test function that needs it. The resulting test function code then looks like this:

Here monkeypatch.setitem() will memorize old settings and modify the environment. When the test function finishes the monkeypatch object restores the original setting. This test function is free to get moved across files. No other test function or code place is affected or required to change when it moves.